1. Field of the Invention
The present invention relates to liquid crystal displays having discharge tube lamps. More particularly, the present invention relates to driving such lamps.
2. Description of the Related Art
Because liquid crystal displays (LCDs) are often lightweight and slim, and have low power consumption, LCDs are often used in office automation equipment, audio/video equipment, and as computer displays. LCDs operate by controlling the amount of light that is transmitted through a liquid crystal based on video signals applied to a plurality of control switches. The video signals enable LCD devices to display desired images on a screen.
To display images, LCD devices typically require a backlight as a light sources. For example, cold cathode fluorescent tubes (CCFL) are commonly backlights. CCFL sources operate according to the cold emission phenomenon, i.e., electron emission caused by a strong electric field applied to the surface of a cathode. CCFL sources provide low heat emission, high brightness, long life, and can produce full color images. CCFL sources may be used in light guide systems, direct illumination systems, and reflection plate systems. Thus, CCFL are readily adopted to the needs a particular application.
CCFL sources are driven by high voltage AC power. High voltage AC power is obtained by boosting AC power using piezoelectric transformers (not shown) after DC power from a DC power supplier (not shown) has been converted to AC power by inverters (not shown).
Inverters that supply AC power to CCFL sources may be classified as either Continuous Mode Inverters or Burst Mode Inverters.
Continuous Mode Inverters supply a continuous AC waveform to CCFL sources. Burst Mode Inverters periodically supply Pulse Width Modulation (PWM) controlled AC waveforms to CCFL sources.
Referring to FIG. 1, the brightness of a CCFL source when driven by a Continuous Mode Inverter is proportional to the amplitude (a) of the electric power. As illustrated in FIG. 2, Continuous Mode Inverter output waveforms appear to be concentrated within a spectrum (A) comprised of a single frequency (f0) when converted by the Fourier Transform, reference equation 1.
                              f          ⁢                                          ⁢                      (                                                  ⁢            t            )                          =                                            a              0                        2                    +                                    ∑                              n                -                0                            ∞                        ⁢                                          A                n                            ⁢                              cos                ⁡                                  (                                      n                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                                    )                                                                                        (        1        )            f(t)=A cos(nω0t)  (2)
In the present example, ωo, the angular frequency=2π/to, n=the harmonic value, the Fourier Coefficients a0=0 (dc component of the output waveform), and An=A (amplitude of the output waveform).
The Fourier Coefficients a0 and An may be substituted into the Fourier series of equation 1 to provide the resultant equation 2 that describes the frequency spectrum shown in FIG. 2. Continuous Mode Inverters supply stable output waveforms with little loss when inducing AC gas discharges within CCFL sources.
Continuous Mode Inverters continuously supply AC power to CCFL sources, even when the liquid crystal panel is not driven. Accordingly, Continuous Mode Inverters cause CCFL sources to consume a relatively large amount of power. Also, the brightness adjustment range of the CCFL source is narrow because the adjustment range is dependent upon the amplitude (a) of the output waveform supplied from the inverter.
One approach to overcoming the deficiencies of CCFL sources driven by Continuous Mode Inverters is to use Burst Mode Inverters.
The Burst Mode Inverter can reduce power consumption and provide greater control of the brightness of CCFL sources. Referring to FIG. 3, a duty-on-time (duty cycle), τ, of the output waveform from a Burst Mode Inverter may be expressed as a ratio of, and may be adjusted relative to, a predetermined Pulse Width Modulated (PWM) frequency, Tp. While strictly speaking Tp is a time period, since time periods are reciprocals of frequencies, the following refers to PWM frequencies, thus following common terminology. The PWM frequency thus corresponds to a sequence of time periods in which a transistor (not shown) disposed between the DC power source and the inverter can be turned on and off. The PWM frequency thus controls the switching times that DC power supplied from the DC power source can be switched by the inverter. The duty-on-time (or duty cycle) corresponds to the amount of time in which the transistor is turned on during one period of the PWM frequency.
The output waveform of the AC power supplied from the Burst Mode Inverter may be converted by a Fourier Transform to produce the frequency spectrums illustrated in FIGS. 4 and 5.
Since the Burst Mode Inverter output waveform provides power to the CCFL source in accordance with the ratio of the duty-on-time with respect to the PWM frequency, harmonic components provide the frequency range required to drive the CCFL source.
FIG. 4 illustrates a frequency spectrum of the Burst Mode Inverter output waveform defined by equation 1, wherein the ratio of the duty-on-time to one period of the PWM frequency Tp is 1:5, i.e., the duty-on-time is 20% of one period of the PWM frequency. Fourier coefficients in the above equation 1 may be briefly defined as shown below:
                              a          0                =                                            A              ⁢                                                          ⁢              τ                        Tp                    =                      c            ⁢                                                  ⁢            component                                                            a          n                =                                            2              Tp                        ⁢                                          ∫                0                T                            ⁢                                                          ⁢                              A                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              n                                                  t                          Tp                                                                                      )                                                  ⁢                                  ⅆ                  t                                                              =                                    A                              π                ⁢                                                                  ⁢                n                                      ⁢                          sin              ⁡                              (                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  n                  ⁢                                      t                    Tp                                                  )                                                                                      b          n                =                                            2              Tp                        ⁢                                          ∫                0                T                            ⁢                              A                ⁢                                                                  ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                                              n                                                  t                          Tp                                                                                      )                                                  ⁢                                  ⅆ                  t                                                              =                                    A                              π                ⁢                                                                  ⁢                n                                      ⁢                          (                              1                -                                  cos                  ⁡                                      (                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      n                      ⁢                                              t                        Tp                                                              )                                                              )                                                              [                              n            =            1                    ,          2          ,          3          ,          4          ,                                          ⁢          …                ]            
The fourier coefficients shown above may be alternately expressed in plural exponent format, as illustrated below:
      c    n    =                    1        Tp            ⁢                        ∫          0          T                ⁢                  A          ⁢                                          ⁢                      ⅇ                                          -                j                            ⁢                                                          ⁢              n                                ⁢                                    2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              n                        Tp                    ⁢                      ⅆ            t                                =                            A                      j            ⁢                                                  ⁢            n            ⁢                                                  ⁢            2            ⁢            π                          ⁢                  (                      1            -                                          ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                  n                                            ⁢                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  τ                                Tp                                              )                    =                                                  A              ⁢                                                          ⁢              τ                        Tp                    ⁢          e                -                  jn          ⁢                      πτ            Tp                    ⁢                                    sin              ⁡                              (                                                      n                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    τ                                    Tp                                )                                                                    n                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                τ                            Tp                                          
Further, the Fourier coefficients expressed in plural exponent form as shown above may be briefly defined as illustrated below:
                              c          0                =                ⁢                              A            ⁢                                                  ⁢            τ                    Tp                                                                        c            0                                    =                ⁢                                            A              ⁢                                                          ⁢              τ                        Tp                    ⁢                                                                sin                ⁡                                  (                                                            n                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      τ                                        Tp                                    )                                                                              n                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  τ                                Tp                                                                                        [                              n            =                        ⁢            1                    ,          2          ,          3          ,          …                ]            
The magnitude of the spectrums illustrated in FIGS. 4 and 5 are proportional to the duty-on-time (duty cycle) and are inversely proportional to the PWM frequency. Differences in frequencies of each spectrum are inversely proportional to the PWM frequency.
Referring to FIG. 4, the index of dispersion, R, is defined as the ratio of the frequency magnitude at the central region of the frequency spectrum having the largest value, f0, to the frequency magnitude of a first harmonic, f1, most closely adjacent to spectrum at f0, as shown in equation 3, below.
                              index          ⁢                                          ⁢          of          ⁢                                          ⁢          dispersion          ⁢                                          ⁢                      (            R            )                          =                                            f              1                                      f              0                                =                      93            ⁢            %                                              (        3        )            Accordingly, the index of dispersion represents the concentration of spectrum harmonics within a specific frequency range. As the index of dispersion decreases, the concentration of spectrum harmonics in a specific frequency range increases. Likewise, as the index of dispersion increases, the concentration of spectrum harmonics in a specific frequency range decreases.
Still referring to FIG. 4, with an index of dispersion of 93%, the Burst Mode Inverter output waveform supplied to the CCFL source is dispersed over many frequency ranges. Accordingly, electric current components within the output waveform enable the CCFL source to produce a stable glow discharge.
The electric current components within the output waveform do not affect the normal glow discharge inside the CCFL source nor are they consumed in any space discharge between the CCFL source and its surroundings (such as lamp housing and space which are not shown). However, a space discharge phenomenon is prevalent near the high voltage terminal of CCFL source, to which AC power is supplied. Briefly, the space discharge phenomenon occurs when mercury, Hg, ionized within the CCFL source, migrates on a large scale from the high voltage terminal towards a low voltage terminal of the CCFL source.
If the CCFL source is driven for long periods of time, the space discharge phenomenon is also sustained for long periods of time. Accordingly, all of the mercury atoms inside the CCFL source undergo large scale migration from the high voltage terminal toward the low voltage terminal. To emit light, mercury undergoes UV light emitting chemical reactions including ionization, excitation, etc. Since mercury within the CCFL source migrates towards the low voltage terminal, more mercury is accumulated near the low voltage terminal of the CCFL tube compared to mercury accumulation near the high voltage terminal. Accordingly, a deviation in the brightness within the CCFL tube results and the high voltage terminal of the CCFL source appears to be darker than the low voltage terminal.
FIG. 5 illustrates a spectrum of the Burst Mode Inverter output waveform defined by equation 1, wherein the ratio of the duty-on-time to one period of the PWM frequency is 3: Tp, i.e., the duty-on-time is 34% of one period of the PWM frequency.
The index of dispersion in the frequency spectrum of the output waveform shown in FIG. 5, calculated similarly with respect to the frequency spectrum in FIG. 4, is 83%. With an index of dispersion of 83%, the Burst Mode Inverter output waveform supplied to the CCFL source is dispersed over many frequency ranges. Accordingly, electric current components within the output waveform enable the CCFL source to produce a stable glow discharge.
The electric current components within the output waveform do not affect the normal glow discharge inside the CCFL nor are they consumed in any space discharge between the CCFL and its surroundings (such as lamp housing and space which are not shown). However, the space discharge phenomenon is prevalent near the high voltage terminal of the CCFL where mercury, Hg, ionized within the CCFL sources moves from the high voltage terminal to a low voltage terminal of the CCFL source. As described above, a deviation in brightness occurs in the CCFL source.
As the duty-on-time ratio decreases, power consumption is of the CCFL source is reduced. However, as described above, mercury migration inside CCFL sources deleteriously reduces its operational life. Additionally, as the PWM frequency increases, the CCFL source become less reliable as it may not turn on because the electric current required to produce discharge is not adequately supplied.